As a drill for drilling holes, there are for example those in which an inner insert and an outer insert are detachably attached to the tip end of a holder so that their respective rotation loci are partially overlapped with each other. Among others, those in which the inner insert and the outer insert have the same shape are frequently used. That is, the drill in which one type of drill insert (hereinafter referred to as “insert” in some cases) is detachably attached to each of the inner side and the outer side at the tip end of the holder is frequently used.
The inserts used for this drill include an inner cutting edge and an outer cutting edge. The inner cutting edge is the cutting edge for mainly cutting (machining) an inner portion of a bottom face of a hole when it is used as the inner insert. The outer cutting edge is the cutting edge for mainly cutting an outer portion of a bottom face of a hole when it is used as the outer insert.
For example, the insert described in Japanese Unexamined Patent Application Publication No. 10-180521 has the inner cutting edge and the outer cutting edge adjacent to each other which are formed at the intersection portion between the upper face and the side face. Breaker grooves for treating chips are formed along both cutting edges in the upper face.
One of these inserts and the other are respectively attached as the inner insert and the outer insert to insert pockets formed at the tip end portions of a substantially columnar holder. The hole drilling of a work material is carried out with both cutting edges by rotating the holder around the axis of the holder. The chips generated during the hole drilling are treated through the breaker grooves of the inserts. These breaker grooves are formed in substantially the same shape over the entire periphery of the inserts.
However, the rotational speed of the inner cutting edge and the outer cutting edge differ in rotational speed. Therefore, the chip shape generated by the inner cutting edge and the chip shape generated by the outer cutting edge usually differ widely from each other. That is, the chips generated by the inner cutting edge have a spiral three-dimensionally complicated shape. The chips generated by the outer cutting edge have a spring-like curled shape.
Particularly, when machining a work material having excellent ductility, such as stainless steels or low carbon steels, the chips generated by the outer cutting edge under high rotational speed are unsusceptible to curling, so that they are likely to extend without being cut and likely to cling to the holder during the machining. There has been the problem that these chips cannot be smoothly discharged through the breaker grooves.
For example, the insert described in Japanese Unexamined Patent Application Publication No. 2001-252809 includes an inner cutting edge, an outer cutting edge, a recess shape breaker groove formed along the outer cutting edge, and a raised part formed along the breaker groove. The raised part is located at a higher position than the upper face located along inner cutting edge.
According to this insert, the chips generated by the outer cutting edge under high speed rotation can be curled and cut by the breaker grooves and the raised part, thus achieving improvement of chip discharge performance. It is hence considered that in order to machine the work material having excellent ductility, the chips generated by the outer cutting edge may be further curled and the height of the raised part may be further increased therefor.
However, there has been the problem that merely increasing the height of the raised part increases the level difference between the raised part and the upper face along the inner cutting edge, thereby making it easy for the chips to accumulate at the level difference portion. Particularly, there has been a noticeable problem that the chips having a three-dimensionally complicated shape generated by the inner cutting edge accumulate at the level difference portion of the inner cutting edge located at the outer periphery side of the inner cutting edge which corresponds to a discharge direction in which the chips generated by the inner cutting edge are discharged outside of the holder.
That is, there has been the problem that a further increase in the height of the raised part for improving the discharge performance of the chips generated by the outer cutting edge contributes to improving the discharge performance of the chips generated by the outer cutting edge, while deteriorating the discharge performance of the chips having the three-dimensionally complicated shape generated by the inner cutting edge.